Electric resistance welder and method

ABSTRACT

Resistance welder supplies an AC preheat current to the workpiece, followed by a DC weld pulse.

United States Patent Frless 1 Apr. 25, 1972 541 RESISTANCE WELDER AND[56] References Cited UNITED STATES PATENTS [72] hard 2,306,229 12/1942Somerville ..219/117 x 73 Assignee: Hughes Aircraft Company, CulverCity, 2,066,668 1/1937 Bennett ..219/50 c6111. 2,392,101 l/1946 Ringer..219/116 2,826,674 3/1958 Peras ..219/111x [221 Apr-13,1970 3,408,47310/1968 Gilbert ..219/111 [21] APPLNOJ 27,888 3,436,514 4/1969Broomhalletal. ..219 113 Primary Examiner-J. V. Truhe 521 u.s.c1..219/1ll,2l9/86,2l9/ll5 4mm"! Schutzman 51 1111.01 .3231111/24may-James!flaske'land Alla" A-Dicke 5s FieldofSearch ..2l9/l08-l|6,86,

2mm 57 ABSTRACT Resistance welder supplies an AC preheat current to theworkpiece, followed by a DC weld pulse.

8 Claims, 2 Drawing Figures 4,6 Cox 7K0; Z0 fzz C/lcd/f O- O 11:, oflat/mew y if:

PATENTEU m 2 5 m2 SHEET 2 CF 2 i i l 1 NW m m I u M M u n A L 3 H-N\ v9w av T u m w n K n F F M IIIIMWIIIII: l I W l H uh u T H M wzw u+ I((1y ELECTRIC RESISTANCE WELDER AND METHOD BACKGROUND This invention isdirected to a resistance welder which sequentially supplies AC and DCcurrent to the workpiece for preheating and welding.

The employment of alternating current for resistance welding, or spotwelding, is well-known. A considerable amount of such equipment isavailable in the marketplace. An alternating current pulse train ofsufiicient magnitude and sufiicient duration to cause resistance heatingof metal parts lying next to each other causes melting of the facingsurfaces. Despite careful control to provide adequate but not excessivepower, such alternating current welding tends to leave a burned areaaround the weld spot.

Also known in the art are direct current welders which supply a singledirect current pulse of sufficient energy to melt the adjoining faces ofmetal parts to be welded, with consequent joining. The large pulse in DCwelding sets up large local magnetic fields which tend to spatter liquidmetal from the pool which has been melted by the pulse. in both thecases of AC and DC welding, reducing the power to the minimum valuewhich produces a satisfactory weld still causes these undesirablefeatures.

SUMMARY In order to aid in the understanding of this invention, it canbe stated in essentially summary form that it is directed to a methodand apparatus for the welding of adjoining metal parts by the sequentialapplication of an AC train of sufficient magnitude and duration topreheat the parts preparatory to welding, followed by the application ofa DC pulse of sufficient energy to cause welding of the parts.

Accordingly, it is an object of this invention to provide an AC-DCwelder for the welding of metal parts lying in association with eachother. It is a further object to provide a welder which supplies an ACsignal through the associated parts of sufficient power to preheat theparts, followed by a DC pulse of sufficient power to cause melting ofthe associated parts to cause welding therebetween. It is another objectto provide a combination AC-DC welder where the AC pulse is insufficientto cause welding, and the following DC pulse is of insufficient energyto cause welding, if the parts are not preheated to thus avoid burningadjacent the weld area and to avoid splashing of the weld-metal pool. ltis still another object to provide an AC-DC welder which is economic ofconstruction and yet produces optimum welds of adjacent metallicworkpieces. Other objects and advantages of this invention will becomeapparent from a study of the following portion of the specification, theclaims and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram ofthe welder of this invention.

FIG. 2 is an electrical schematic diagram of the welder of thisinvention.

DESCRIPTION Referring to the drawings, the weld control circuit of thewelder of this invention is illustrated as being comprised of anactuator switch 12, which is connected to selector switch 14, see FIG.2, which is, in turn, connected to the AC duration control circuit 16.One output of the AC duration control circuit 16 goes to AC controlcircuit 18 which has its output to AC primary 20 of weld transformer 22.Weld transformer 22 is of such nature as to have primary inputs ofrelatively high voltage and low current which produce outputs insecondary 24 of fairly low voltage and high current. Connected to thesecondary 24 are welding electrodes 26 and 28 which are adapted to bemoved against workpieces 30 and 32. As in conventional practice, thewelding electrodes engage the workpieces with sufficient force toprovide adequate electrical contact and, in some cases, to providesufficient clamping force to aid in producing inter-workpiece engagementfor enhancing both electrical contact therebetween and maintainingclamping during the weld heating and cooling cycle. Such is available inconventional welders.

Another output of the AC duration control circuit 16 goes to delaycircuit 34 which, in turn, has its output through selector switch to DCcontrol circuit 36. The output of the DC control circuit 36 goes to asecond primary 37 on weld transformer 22.

Selector switch 14 has two switch arms which are connected together tomove together, and each of the switch arms has three positions. As isseen in FIG. 2, in the upper position, actuation of switch 12 actuatesonly the AC welder. In the bottom position, actuation of switch 12actuates only the DC welder portion of the circuit. The central positionof the selector switch 14 is connected so that closure of actuatorswitch 12 causes the circuitry to first actuate the AC welder portion ofthe circuit for an appropriate period at an appropriate amplitudefollowed, after suitable delay, by actuation of the DC control circuit.AC duration control circuit 16 is provided with a positive power line38. Transistor 40 is connected between the positive power line and theground through collector and emitter resistances. its base is connectedto the positive power line 38 through variable resistance 42 which actsto control the duration of the AC cycle. Since the base is so connected,until actuator switch 12 is closed, which is also connected to the baseof transistor 40, the base voltage builds up. With the base built up involtage substantially to the voltage of positive power line 38, thetransistor 40 is on so that its collector is substantially at groundvoltage. When actuator switch 12 is closed, a negative-going pulse isconveyed through capacitor 44 to the base of transistor 40 to render thetransistor 40 nonconductive.

Transistor 46 has its collector connected through a collector resistanceto the DC positive power line 38 and has its emitter connected to theemitter of transistor 40. Thus, when transistor 40 changes from itsquiescent conductive state to its actuated nonconductive state, theemitter voltage on transistor 46 goes down. At the same time, since thebase of transistor 46 is connected to the collector of transistor 40,the base voltage on transistor 46 goes up to result in tumon oftransistor 46. This results in collector line 48 going toward zero.Timing capacitor 50 is connected between collector line 48 and the baseof transistor 40. When collector line 48 goes down in voltage, capacitor50 is discharged and can be charged through resistance 42. Thus, thevalue of the variable resistor 42 controls the charging time ofcapacitor 50. When the capacitor 50 charges to an adequate value, itsconnection to the base of transistor 40 causes transistor 40 to againbecome conductive to signal the end of time of the AC duration.

Conventional alternating current supply 52 (such as a l 15 volt, 60cycle line) supplies the primary of control circuit power supplytransformer 54. The output of the transformer 54 is rectified byconventional fullwave rectifier diodes 56 to supply filtered current topositive power line 38, which is controlled by zener regulator 58. Therectified current from the power transformer 54 is also supplied throughunfiltered line 60, which is clipped at maximum voltage by zenerregulator 62. Thus, the voltage in line 60 has a maximum positivevoltage limit with downwardly-directed spikes reaching zero volts. Thesespikes are synchronized with the input AC voltage of supply 52.

Collector line 48 is connected to the base of transistor 64, which hasits emitter connected to ground and its collector connected to line 60through variable resistance 66. The collector of transistor 64 also isconnected to capacitor 68, which has its other side connected to groundto form an RC network. The collector of transistor 64 is connected tothe emitter of unijunction transistor 70. Unijunction transistor 70 hasits base 2 connected through a resistor to positive supply line 60 andhas its base 1 connected to ground through the primary of pulsetransformer 72. I v

Since the voltage of line 48 has been high, before closure of actuatorswitch 12, transistor 64 has been conducting. When the AC durationcircuit was energized, the voltage in line 48 decreased, to cut offtransistor 64. This permits the current flow from line 60 throughresistor 66 to charge capacitor 68, increasing the emitter voltage onunijunction transistor 70. The ramp slope is a function of setting ofvariable resistor 66, so that oscillation rate is controlled. When theemitter of unijunction transistor 70 reaches an adequate voltage,usually about one-half the voltage between base 1 and base 2, theunijunction transistor becomes conductive from emitter to base 1.Discharge of capacitor 68 causes a pulse through pulse transformer 72.This pulse is in synchronism with the AC power line so that the pulsetransformer can serve as a gate connection to SCR 74.

SCR 74 is serially connected with the AC power source and with primaryon weld transformer 22. Thus, the point at which SCR 74 is fired on itswave is a function of the setting of resistance 66. Furthermore, SCR 74fires at every positive going wave until the AC duration control circuit16 times out, and the line 48 becomes more positive to cause conductionof transistor 64.

To accomplish the DC discharge to the weld transformer, it is desirableto delay firing of the DC control circuit for a short while, in order toprevent firing of the DC while the transformer is saturated fromprevious AC cycling. Delay circuit 34 is shown as an adjustable delayone-shot multivibrator, identical to the AC duration control circuit 16.The reason for delay is that, if no delay has been provided, SCR 74 mayhave just been fired to saturate weld transformer 22, so that, with animmediate firing of the DC pulse, may result in firing it into asaturated transformer. Thus, delay to permit the desaturation of thetransformer is necessary to make sure that the sequential operation isfully effective. Thus, any delay circuit can be substituted for thedelay circuit 34, providing it is of such nature as to provide ahalf-cycle delay after timing out of control circuit 16.

Referring to FIG. 2, the delay circuit 34 is identical to the ACduration circuit 16. When the AC duration circuit 16 times out, line 76connected between the collector of transistor and the circuit 34 has anegative-going spike which, similarly to the previously-describedcircuit 16, causes transistor 78 to be nonconductive for a perioddetermined by variable resistor 80 and, when it again goes conductive, anegative-going spike is produced in line 82 which causes actuation ofthe DC control circuit 36.

The DC control circuit 36 is indicated in block diagram in FIG. 1 and isindicated generally by that number in FIG. 2. Transformer secondary 84is energized from any convenient AC source, such as that supplied totransformer 54. Transformer secondary 84 is center-tapped to providenegative bus 86, which is conveniently at ground potential. Transformersecondary 84 has its ends connected to diodes 88 and 90, which are bothconnected to unregulated positive bus 92. SCR 94 is connected betweenunregulated positive bus 92 and regulated positive bus 96. SCR 94 hasits gate connected to charge control circuit 98. The charge controlcircuit receives its signal from the unregulated bus, the regulated busand the ground potential. Additionally, line 82 is connected to weldcontrol circuit 100, which has a signal line 102 connected to aid in thecontrol of the charge control 98. The charge control circuit isconventional, or could be that which is shown in U.S. Pat. applicationSer. No. 819,8321, filed Apr. 28, 1969, by Malcolm M. Oakes, entitledCapacitor Charge Circuit." The weld control circuit 100 is conventionalor could be that disclosed in U.S. Pat. application Ser. No. 14,421,filed Feb. 26, 1970, by Richard G. Friess and Tor Hougcn, entitled SCRTum-Off Circuit.

Capacitor 104 is connected between regulated positive bus 96 andnegative bus 86. The purpose of SCR 94, and its charge control 98, is tocharge up the capacitor 104 to a predetermined value. The charge controlregulates the SCR to accomplish this purpose. Secondary 37 is connectedin series with SCR 106 and between buses 96 and 86. Thus, when SCR 106is fired by weld control 100, the capacitor 104 discharges throughprimary 37 to causes a DC weld pulse between electrodes 26 and 28.

Asa specific example of the values of the elements of the circuit, Tablel is presented below. The values of the elements which are not recitedare suitably compatible.

TABLE I Reference ltem Identification No. 22 Weld Transformer 24 to 1Turns Ratio on both primaries 38 Positive Power Line +16 volts 40Transistor 2N 27 12 42 Variable Resistance 2K-52K ohms 46 Transistor 2N27l2 50 Capacitor l0 Microfarads 56 Diodes SCE 2 58 Zener diode 18 volts62 Zener diode 18 volts 64 Transistor 2N 27l2 66 Variable Resistor839K-89K ohms 68 Capacitor 0.1 Microfarads 70 Unijunction Transistor 2Nl67l 72 Pulse Transformer l-l turns ratio 74 SCR 2N 398 78 Transistor 2N2712 80 Variable Resistor 2K-22K ohms 88 Diode ISA-600 V) such as lN32l490 Diode l5A-600 V) 94 SCR 2N 3898 I04 Capacitor 6 250 mfd 0 450 V 106SCR 2N 3898 In operation, the workpieces 30 and 32 are placed betweenthe electrodes and selector switch 14 is set for the desired function.Assuming that the AC input to electrodes 26 and 28 is first desired,followed after an interval by a DC pulse, selector switch 14 is set inthe middle position, shown. The duration of the AC signal is set onvariable resistor 42 and the amplitude of the signal is set on variableresistor 66. Variable resistor 80 is set to determine the amount of timedelay between the end of the AC input and the firing of the DC pulse.After the settings are accomplished, actuator switch 12 is closed. Thiscauses firing of SCR 74 at the proper point in the AC cycle to providethe desired amplitude, the pulse train continuing for the duration, asdetermined by the setting of resistor 42. After the pulse train isterminated, at conclusion of the AC signal, delay circuit 34 delays theDC firing for an ap propriate length of time. Thereupon, SCR 106 is madeconductive by weld control to cause the DC weld pulse.

As described above, the use of both AC and DC, in that sequence, permitspreheating of the workpieces by the AC signal, at a level below thatwhich would cause a weld, followed by a DC weld pulse which would beinadequate to cause a weld, if preheating had not been employed. Weldingany two pieces of metal together requires sufficient energy in the weldpulse to flow the metal but, in many cases, before this energy level canbe reached, the bad effects start interfering, such as splattering awaythe metal or burning. This system simply allows bonding below these highenergy levels which produce the unwanted afl'ects. An example of weldingfor which this welder and method is particularly useful is welding afine wire to a more massive body. A too high energy pulse will splatteraway the molten material and leave a crater effect on each side of thewire. As a result, little contact area is achieved and this isdetrimental when passing high currents through the wire or wherestructural strength is desired.

This invention having been described in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art and without the exerciseof the inventive faculty.

What is claimed is:

1. A welder for spot welding together weldable material, said weldercomprising:

first and second electrode means for engagement on the weldable materialfor the passage of current through the weldable material for the weldingthereof;

an alternating current weld means connected to said first and secondelectrodes for the passage of a plurality of electrical pulses betweensaid first and second electrodes for a sufficient time and at asufficient amplitude to preheat the weldable material;

direct current weld means connected to discharge direct current betweensaid first and second electrodes and through the workpiece for weldingthe weldable material; and

control means for sequentially activating said alternating current weldmeans and said direct current weld means, said control means includingdelay means to delay the start of direct current flow between said firstand second electrodes for a predetermined time after cessation of theelectrical pulses from said alternating current weld means so that theelectrical pulses from said alternating current weld means cause preheatof the weldable material and the direct current through the weldablematerial causes welding thereof.

2. The welder of claim 1 wherein said control means includes means forcontrol of the duration of the electrical pulses from said alternatingcurrent weld means, said means for control of the duration of theelectrical pulses from said alternating current weld means comprises aone-shot multivibrator with an adjustable time constant.

3; The welder of claim 2 wherein said delay means for delaying thedirect current until after cessation of the electrical pulses from saidalternating current weld means comprises a one-shot multivibrator havingan adjustable time device.

4. The welder of claim 1 wherein said welder includes a transformersecondary having said first and second electrodes in series therewith,first and second transformer primaries in electromagnetic coupling withsaid secondary, said first transformer primary being energized by saidalternating current weld means and said second transformer primary beingenergized by said direct current weld means.

5. A welder for spot welding together weldable material, said weldercomprising:

first and second electrodes for engagement on the weldable material, atransformer secondary connected in series with said first and secondelectrodes for the passage of current through the weldable material forthe welding thereof;

an alternating current weld means including a first transformer primaryin electromagnetic coupling 6. The welder of claim 5 wherein saidcontrol means includes means for control of the duration of the AC pulsetrain, said means for control of the duration of the electrical pulsesfrom said alternating current weld means comprises a oneshotmultivibrator with an adjustable time constant.

7. The welder of claim 6 wherein said delay means for delaying thedirect current until after cessation of the electrical pulses from saidalternating current weld means comprises a one-shot multivibrator havingan adjustable time device.

8. A welder for spot welding together weldable material, said weldercomprising:

means for engagement on the weldable material for the passage ofelectric current through the engagement means and through the weldablematerial for the welding thereof, said means for engagement comprisingfirst and second electrodes;

means connected to said first and second electrodes for the passage of aplurality of electrical pulses between said first and second electrodesfor a sufficient time and at a sufficient amplitude to preheat theweldable material, but insufficient to cause welding thereof, said meanscomprising alternating current source weld means; means connected tosaid first and second electrodes for discharging direct current betweensaid first and second electrodes and through the workpiece withsufficient energy for welding the weldable material when it is preheatedby said alternating current source weld means, and insufficient to causewelding when not preheated by said alternating current source weldmeans; and

means for sequentially activating first said alternating current sourceweld means and subsequently said direct current weld means while theweldable material is still preheated, including means for delaying theactivation of the start of direct current discharge between saidelectrodes from said direct current weld means for a predetermined timeafter cessation of the electrical pulses from said altemating currentsource weld means, so that the electrical pulses from said alternatingcurrent source weld means cause preheat of the weldable material and thedirect current through the weldable material causes welding thereof.

29x3 UNITED S'IA'II'JS IA'IENI OFFICE CERTIFICATE OF CORRECTION PatentNo. 316591074 D t d p i 1972 lnventofls) Richard G. Friess It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 4, line 33, delete "mfd 0" and substitute mfd (page 10, line 24)Column 6, after line 4, insert with said transformer secondary, saidfirst transformer primary being energized by said alternating currentweld means for the passage of a plurality of electrical pulses betweensaid first and second electrodes for a sufficient time and at asufficient amplitude to preheat the weldable material; I V

direct current weld means including a second transformer primary inelectromagnetic coupling with said transformer secondary connected todischarge direct current between said first and second electrodes byenergization of said direct current weld means for welding of theweldable material; and

control means including delay means to delay the start of direct currentflow between said first and second electrodes for a predetermined timeafter cessation of the electrical pulses from said alternating currentweld means for sequentially activating said alternating currentweldmeans and said direct current weld means so that the electricalpulses from said alternating current weld means cause preheated weldablematerial and the direct current through the weldable material causeswelding thereof.

Signed and sealed this 27th day of March 1973 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents 29x3 UNITED STATES mm OFFICE CERTIFICATE OF CORRECTION PatentNo, 316591074 D d April 25, 1972 Invenmfls) Richard G. Friess It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 4, line 33, delete "mfd O" and substitute mfd (page 10, line 24)Column 6, after line 4, insert with said transformer secondary, saidfirst transformer primary being energized by said alternating currentweld means for the passage of a plurality of electrical pulses betweensaid first and second electrodes for a sufficient time and at asufficient amplitude to preheat the weldable material; I v

direct current weld means including a second transformer primary inelectromagnetic coupling with said transformer secondary connected todischarge direct current between said first and second electrodes byenergization of said direct current weld means for welding of theweldable material; and

control means including delay means to delay the start of direct currentflow between said first and second electrodes for a predetermined timeafter cessation of the electrical pulses from said alternating currentweld means for sequentially activating said alternating currentweld'means and said direct current Weld means so that the electricalpulses from said alternating current weld means cause preheated weldablematerial and the direct current through the weldable material causeswelding thereof.

Signed and sealed this 27th day of March 1973..

(SEAL) Attest:

EDWARD M.FLETCHER,JR. I ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents 3

1. A welder for spot welding together weldable material, said weldercomprising: first and second electrode means for engagement on theweldable material for the passage of current through the weldablematerial for the welding thereof; an alternating current weld meansconnected to said first and second electrodes for the passage of aplurality of electrical pulses between said first and second electrodesfor a sufficient time and at a sufficient amplitude to preheat theweldable material; direct current weld means connected to dischargedirect current between said first and second electrodes and through theworkpiece for welding the weldable material; and control means forsequentially activating said alternating current weld means and saiddirect current weld means, said control means including delay means todelay the start of direct current flow between said first and secondelectrodes for a predetermined time after cessation of the electricalpulses from said alternating current weld means so that the electricalpulses from said alternating current weld means cause preheat of theweldable material and the direct current through the weldable materialcauses welding thereof.
 2. The welder of claim 1 wherein said controlmeans includes means for control of the duration of the electricalpulses from said alternating current weld means, said means for controlof the duration of the electrical pulses from said alternating currentweld means comprises a one-shot multivibrator with an adjustable timeconstant.
 3. The welder of claim 2 wherein said delay means for delayingthe direct current until after cessation of the electrical pulses fromsaid alternating current weld means comprises a one-shot multivibratorhaving an adjustable time device.
 4. The welder of claim 1 wherein saidwelder includes a transformer secondary having said first and secondelectrodes in series therewith, first and second transformer primariesin electromagnetic coupling with said secondary, said first transformerprimary being energized by said alternating current weld means and saidsecond transformer primary being energized by said direct current weldmeans.
 5. A welder for spot welding together weldable material, saidwelder comprising: first and second electrodes for engagement on theweldable material, a transformer secondary connected in series with saidfirst and second electrodes for the passage of current through theweldable material for the welding thereof; an alternating current weldmeans including a first transformer primary in electromagnetic coupling6. The welder of claim 5 wherein said control means includes means forcontrol of the duration of the AC pulse train, said means for control ofthe duration of the electrical pulses from said alternating current weldmeans comprises a one-shot multivibrator with an adjustable timeconstant.
 7. The welder of claim 6 wherein said delay means for delayingthe direct current until after cessation of the electrical pulses fromsaid alternating current weld means comprises a one-shot multivibratorhaving an adjustable time device.
 8. A welder for spot welding togetherweldable material, said welder comprising: means for engagement on theweldable material for the passage of electric current through theengagement means and through the weldable material for the weldingthereof, said means for engagement comprising first and secondelectrodes; means connected to said first and second electrodes for thepassage of a plurality of electrical pulses between said first andsecond electrodes for a sufficient time and at a sufficient amplitude topreheat the weldable material, but insufficient to cause weldingthereof, said means comprising alternating current source weld means;means connected to said first and second electrodes for dischargingdirect current between said first and second electrodes and through theworkpiece with sufficient energy for welding the weldable material whenit is preheated by said alternating current source weld means, andinsufficient to cause welding when not preheated by said alternatingcurrent source weld means; and means for sequentially activating firstsaid alternating current source weld means and subsequently said directcurrent weld means while the weldable material is still preheated,including means for delaying the activation of the start of directcurrent discharge between said electrodes from said direct current weldmeans for a predetermined time after cessation of the electrical pulsesfrom said alternating current source weld means, so that the electricalpulses from said alternating current source weld means cause preheat ofthe weldable material and the direct current through the weldablematerial causes welding thereof.